Halep led 6-3, 2-0, then lost a service game for a third time to see the second set back on serve. Mertens continued to pressure, missing two more break chances in the seventh game but finally breaking — to love — in the ninth. The set was the first Mertens took from Halep in three matchups.
Mertens broke again to start the third set, Halep levelled at 2-2, but Mertens finished stronger and won her fifth career title.
The Belgian beat two other top-10 players this week, Kiki Bertens and Angelique Kerber.
Halep's first final appearance since August guaranteed she will rise one spot to No. 2 in the rankings on Monday.